The present invention relates to voltage regulators, and more particularly, to monolithic integrated electronic voltage regulators that can be used in automobile-type applications.
Voltage regulators supply a voltage with a value or values that are well-defined and constant from an unregulated voltage source.
Therefore, they can advantageously be used as regulated power supplies for other devices and, according to the load connected thereto, they supply the necessary current so that the voltage supplied to the load remains constant at all times.
Presently, for reasons of compactness, ease of use and economy, in all fields of application there is a growing tendency toward the production of electronic IC voltage regulators.
As a rule, the voltage across the output terminals of such voltage regulators and their output currents are established by means of an internal regulator circuit which have feedback circuits connected to the output terminals and which are responsive to the instantaneous value of the output voltage and current.
The most commonly used IC voltage regulators are those with a well-defined, series-type, regulation in which the output voltage is adjusted to a constant value by means of a power transistor which is in series with the output and which has its base properly controlled so as to control its conduction as a function of the load.
The basic circuit diagram of such a voltage regulator with series-type regulation is shown in FIG. 1 of the drawings.
A bipolar NPN transistor T.sub.S has its collector and emitter respectively connected to an input terminal IN and an output terminal OUT. Its base is controlled by a differential amplifier A having its supply terminals connected between the input terminal IN and ground. The inverting input of the amplifier A is connected to the output terminal OUT through a resistor R1, and to ground through a resistor R2. The non-inverting input of the amplifier A is connected to a reference voltage V.sub.R.
As is well known to those skilled in the art, a voltage is established between the output terminal OUT and ground whose level depends on the input voltage V.sub.IN and on the load connected to the output terminal OUT only as long as the voltage V.sub.IN does not exceed a predetermined threshold value which is typical of the circuit, above which there is instead established across the output a constant voltage V.sub.O whose value is independent either of the input voltage or of the load and depends only on the design of the circuit proper, in particular on the feedback factor .beta.=R.sub.2 /(R.sub.1 +R.sub.2). In fact, apart from said threshold value, which determines the lower limit of the range for proper operation (and, thus, also for possible use) of the regulator, the regulator circuit operates constantly.
Any deviation of the output voltage from the predetermined value will cause, through the voltage divider R1-R2, a feedback at the inverting input of the differential amplifier A, which drives the transistor T.sub.S to such a level of conduction as to restore a voltage to the load with the predetermined value V.sub.O.
It is desired to identify the operating range, or better still, the lower limit, of a voltage regulator by means of a parameter known in the art as "dropout", which is the difference between the minimum value of the input voltage V.sub.IN necessary for the proper operation of the regulator and the value of the constant voltage V.sub.O established at the regulator output.
The IC voltage regulators normally employed in automobile-type applications are of the above-noted type. However, they must meet very stringent conditions because of the operating conditions characterized either by significant variations in temperature and humidity or by considerable, and sometimes sudden, variations of the supply voltage delivered by the car battery.
Therefore, these regulators must exhibit characteristics of high reliability, accuracy and stability in a very wide operating range together with a very low dropout.
The deviation of the supply voltage normally delivered by the battery can vary from approximately 5.5 V to 6.5 V during a cold start, to approximately 24 V when a second battery is connected in series with the first battery so as to enable an automobile to start under all conditions in cold countries.
However, high voltage surges, both positive and negative, can appear on the power supply line, due to inductive effects (sparking coils, relays, etc.) which occur during turn-off transients. These surges that can reach up to 100 to 120 V due to the accidental detachment of the alternator cable from the battery (in this case, positive surges, with high energies).
In addition to the above mentioned characteristics, a voltage regulator for automobile-type applications must also have a very low power consumption for reasons of efficiency but, above all, for reduced heat dissipation.
The prior art IC voltage regulators are not capable of satisfying all of the conditions for automobile-type applications at the same time. As a matter of fact, properly operating voltage regulators, even for very low input voltages (having, therefore, a low dropout) have a considerable power consumption
At this point, it should be borne in mind that in general, transistors such as T.sub.S depicted in FIG. 1, that are put into integrated circuits carry much higher voltages during turnoff than those carrying voltages during turn-on or when in the active zone.
On the other hand, the regulators that are better from the viewpoint of power consumption have an excessively high dropout for the cold start and are not suitable for proper operation at high voltages.